A well known type of viscosimeter utilizes a cup mounted on a rotating hot plate into which the liquid sample to be tested is introduced and in which a plunger is immersed, the plunger being mounted on the lower end of a torsion wire and spindle assembly suspended at its upper end from a pylon. The spindle is provided with a dial so that as the cup containing the liquid sample is rotated, the spindle and torsion wire assembly rotates also due to the drag of the liquid on the plunger. The spindle rotates until the restoring force of the torsion wire just balances the drag of the liquid and when the dial stops the reading taken therefrom can be converted into centipoises. One such type of viscosimeter is manufactured by the Fisher Scientific Company of Pittsburgh, Pennsylvania.
Such present day apparatus for measuring viscosity is characterized by a number of disadvantages. One fundamental flaw in such a viscosimeter is that the large mass of metal used in the shaft by means of which the plunger is attached to the torsion and spindle assembly removes heat from the rotating cup at such a rate that the resulting thermal gradients set up in the cup make the apparent viscosity readings erratic. The elevated temperature in the cup also causes a rapid transmission of heat to the torsion wire changing the torsional restoring force in the wire and invalidating the low temperature calibration required for accurate viscosity readings. Furthermore, where relatively viscous materials such as polyester resins are tested for viscosity in such an apparatus, the thermal expansion of the resin presented a problem in maintaining a constant immersion level of the plunger in the resin. For instance, as the plunger shaft is provided with a calibration mark which must be observed when making viscosity measurements and when using the small sample adaptor in such an apparatus, maintaining the exact immersion is critical. This is because the diameter of the plunger as it emerges from the liquid is the same as the diameter of the plunger below the liquid level. As a result, it is very difficult to make measurements reproducible to plus or minus 5% even if the calibration mark is clearly visible.
Another problem in the use of the aforementioned prior art apparatus is its limitation as to utility in making room temperature viscosity measurements. It is necessary that the entire spindle and plunger assembly be removed in order to change samples and when fine torsion wire is used, a high incidence of breakage and damage to the torsion wire is present even if extreme care is taken in such a procedure.
Another problem area in such prior art apparatus is in the readout mechanism which utilizes a dial and indicator arrangement. Such a dial arrangement presents a problem in setting the rest point to zero, a very difficult and time consuming procedure especially when fine torsion wire is used. Furthermore, reading the scale on the dial is difficult as a result of poor resolution of the printing on the scale, parallax and the harmonic oscillations of the spindle. Although such prior art apparatus utilizes an oil damper for damping such oscillations, it is cumbersome to use and not very effective. Furthermore, the damper contributes substantially to torsion wire damage.